An organic electro-luminescence display or an organic light emitting display (OELD) generally refers to a flow of electricity in organic material and a light emitting process. The flow of electricity in organic material can be divided into a flow of electrons and a flow of holes. A semiconductor analysis method is generally used because a dominant flow is determined by molecular structures of organic materials.
That is, the flow of electrons or the flow of holes can be dominant according to the molecular structures. The light emitting process is associated with the motion of electrons within molecule. Electrons in the molecule can exhibit a specific energy state such as an excited state, so that electrons hold energy that can be emitted. One aspect of the emission of energy is the observation of light.
In development and application of the organic light emitting display, efficiency is important. Even though a high-brightness device can be fabricated, if the efficiency of the electric energy to optical energy conversion in the device is degraded, an actual application is difficult. Since the organic light emitting display has low power consumption, it is competitive in the markets. Thus, many developments of the organic light emitting display are in progress.
In the organic light emitting device, devices for representing red (R), green (G) and blue (B) colors are separately manufactured. Unlike a TFT-LCD, an organic light emitting device does not use a color filter. That is, RGB colors are reproduced using organic materials that exhibit colors with different brightness according to the applied voltages. Therefore, the organic light emitting device can display images on a screen without using a backlight and a color filter.
The organic materials exhibiting RGB colors have different characteristics according to the applied voltages. That is, brightness characteristics are different according to the applied voltages and efficiency is also different. A driving circuit of a related art organic light emitting display will be described below with reference to the accompanying drawings.
FIG. 1 is a circuit diagram of a driving circuit of a related art organic light emitting display.
Referring to FIG. 1, a PMOS transistor T1 serving as a switching element is arranged in a position where a gate line (GL) and a data line (DL)) are vertically intersected. A gate of the PMOS transistor T1 is electrically connected to the gate line, and a source of the PMOS transistor T1 is electrically connected to the data line.
A drain of the PMOS transistor T1 is electrically connected to a gate of the PMOS transistor T2 that controls a current flowing through an organic light emitting diode (OLED).
A power line arranged parallel to the data line is electrically connected to a source of the PMOS transistor T2. A capacitor Cst is connected between the source and the gate of the PMOS transistor T2 to store a data signal for 1 frame.
A drain of the PMOS transistor T2 is serially connected to one terminal the OLED and another terminal of the OLED is connected to ground.
When a driving signal is applied through the gate line GL, the PMOS transistor T1 connected to the gate line GL is turned on, and data signal is transferred from the source to the drain of the PMOS transistor T1.
Therefore, the data voltage is applied on a node X. Due to the data voltage, a gate-source voltage Vgs is generated in combination with a power supply voltage VDD connected to the source of the PMOS transistor T2 that controls the OLED. The PMOS transistor T2 is controlled by the gate-source voltage Vgs.
That is, while the data voltage Vdata applied to the gate of the PMOS transistor T2 and the power supply voltage VDD are charged in the capacitor Cst for 1 frame, the current flowing through the drain of the PMOS transistor T2 is controlled.
The driving current (I) flowing through the drain of the PMOS transistor T2 is given by a following Equation 1, which is the same equation as for a general field effect transistor (FET).I=K(Vgs−Vth)2  (Equation 1)
where
  K  =            1      2        ⁢    μ    ⁢                  ⁢          Cox      ⁡              (                  W          L                )            
where μ is a mobility, Vth is a threshold voltage of the transistor T2, and Cox is an oxide capacitance, that is, a capacitance for unit area of the gate of the second transistor T2.
Accordingly, the driving current I flowing through the PMOS transistor T2 is controlled by the voltage gate-source voltage Vgs and the power supply voltage VDD. The OLED is controlled by the driving current I.
The driving current of the OLED is derived from the power supply voltage VDD. Therefore, the number of pixels increases, a larger amount of current must be supplied.
For example, when a number of pixels N are provided in a row direction and a full white is driven, the power supply voltage VDD must supply a current (NIpixel). A voltage drop occurs due to line resistance in the VDD supply line (V=IR). That is, the voltage drop in an n-th row is given by[N(N+1)/2]pixel*Ipixel where Rpixel is a line resistance in each pixel and Ipixel is a driving current.
Since the voltage Vgs of the thin film transistor disposed at each pixel is changed due to the voltage drop, a difference of the current in the OLED is caused, depending on the OLED location
The difference of the current applied to the OLED is serious in the large-sized display, causing a non-uniformity of picture quality.